/* 
 * Kodkod -- Copyright (c) 2005-2007, Emina Torlak
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
package kodkod.ast;


import kodkod.ast.visitor.ReturnVisitor;
import kodkod.ast.visitor.VoidVisitor;

/** 
 * An {@link kodkod.ast.Expression expression} with one child.
 * 
 * @specfield expression: Expression
 * @specfield op: Operator
 * @invariant children = expression
 * @author Emina Torlak 
 */
public final class UnaryExpression extends Expression {
    private final Expression expression;
    private final Operator op;
    private final int arity;
    
    /**  
     * Constructs a new unary expression: op expression
     * 
     * @effects this.expression' = expression && this.op' = op
     * @throws NullPointerException - expression = null || op = null
     * @throws IllegalArgumentException - op in {TRANSPOSE, CLOSURE, REFLEXIVE_CLOSURE} && child.arity != 2
     */
    UnaryExpression(Operator op, Expression child) {
        if (!op.applicable(child.arity())) {
            throw new IllegalArgumentException("Invalid arity: " + child + "::" + child.arity());
        }
        this.expression = child;
        this.op = op;
        this.arity = op.arity(child.arity());
    }

    /**
     * Returns the arity of this expression.
     * @return this.arity
     * @see kodkod.ast.Expression#arity()
     */
    public int arity() {
        return arity;
    }
    
    /**
     * Returns this.expression.
     * @return this.expression
     */
    public Expression expression() {return expression;}
    
    /**
     * Returns this.op.
     * @return this.op
     */
    public Operator op() {return op;}
    
    
    /**
     * Accepts the given visitor and returns the result.
     * @see kodkod.ast.Node#accept(kodkod.ast.visitor.ReturnVisitor)
     */
    public <E, F, D, I> E accept(ReturnVisitor<E, F, D, I> visitor) {
		return visitor.visit(this);
	}
   
    
    /**
     * Accepts the given visitor.
     * @see kodkod.ast.Node#accept(kodkod.ast.visitor.VoidVisitor)
     */
    public void accept(VoidVisitor visitor) {
        visitor.visit(this);
    }
    
    /**
	 * Returns the string representation of this expression.
	 * @return string representation of this expression
	 */    
    public String toString() {
        return op.toString() + expression.toString();
    }

    /**
     * Represents a unary expression operator.
     */
    public static enum Operator  {
        /** Transpose (~) operator. */
        TRANSPOSE { public String toString() { return "~";}},
        /** Transitive closure (^) operator. */
        CLOSURE { public String toString() { return "^";}},
        /** Reflexive transitive closure (*) operator. */
        REFLEXIVE_CLOSURE { public String toString() { return "*";}};
        
        /**
         * @return true if this operator can be applied to an expression with the given arity.
         */
        boolean applicable(int childArity) { return childArity == 2; }
        
        /**
         * @return the arity of the expression that results from applying
         * this operator to an expression with the given arity.
         */
        int arity(int childArity) { return 2; }
        
    }
    

}
